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BPU updated on status of chromium plume

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Plume heads towards water supply> Downgradient edge is half-mile from nearest well; interim projects expected to start this year

By Arin McKenna

On Feb. 17, Los Alamos National Laboratory Water Stewardship Program Manager Danny Katzman updated the Board of Public Utilities on the lab’s plans to remediate a chromium plume headed toward the county’s water-supply wells.
The downgradient edge of the plume is a half-mile away from the nearest well. The plume is approximately one mile by one-half mile and located 900–1,000 feet below the canyon bottom, on the top of the aquifer. It is 50-feet thick.
The plume originated from potassium dichromate used in cooling towers at laboratory power plants between 1956 and 1972. Up to 72,000 kilograms was released in hexavalent chromium form (Cr(VI)). The EPA considers hexavalent chromium in groundwater to be a known carcinogenic contaminant.
According to Katzman, much of the 160,000 pounds of chromium released during that period was converted to trivalent chromium in a wetland near the plant.
“(Trivalent chromium) is the same kind of chromium you might find in your multivitamin in the morning,” Katzman said. “It’s essentially harmless, and we view it as essentially out of the equation once it’s move into the trivalent form.”
However, a sizeable portion of hexavalent chromium percolated down to the aquifer. The lab has been monitoring that for about 10 years.
“A couple years ago, we started noticing some pretty important changes,” Katzman said. “The concentrations of chromium, nitrate and percolate – three constituents that are all collocated in the plume – started increasing fairly dramatically in three wells along the plume edge.”
Although one of the monitoring wells closest to the plume shows chromium levels below state standards of less than 50 parts per billion (federal standards are 100 parts per billion), hexavalent chromium has nearly doubled the state standard in one of the wells. A third shows considerable variability with an increasing trend. Nitrate and percolate levels are still well within standard.
The rising chromium levels set plans in motion for a “quick strike action.”
“A strike means you take about a year and a half to get yourself in position to take action,” Katzman said. “We’re there, essentially, right now.”
LANL is working on a two-part plan – one for interim remediation and one for a long-term solution to the problem. Katzman focused on the interim measures underway.  
“We have a very complex fairly long process ahead of us to ultimately remediate this plume,” Katzman said. “This is really complicated work, especially at 1,000 feet depth.”
The interim measure involves hydraulic capture of the chromium plume, treatment in an ion exchange treatment unit and reinjection into the ground water.
The extraction wells will be located at the downgradient edge of the plume rather than the center. The goal is to “grab water that contains chromium and block that water from migrating toward the downgradient edge of the plume.”
When the treated water is injected, it should form a small hydraulic barrier, a mound of water 5−10 feet high that has a “speed bump effect that makes water slow down against it and helps that hydraulic capture.”
The goal is to return water to the aquifer with chromium levels well below state standards.
Katzman addressed concerns that the injection could cause seismic activity similar to that being linked to gas fracking operations. According to Katzman, the water will be returned via gravity fed injection wells.
“The only pressure is basically the amount of water that’s stacked up in the well, producing a bit of pressure on the well itself,” Katzman said. “Plus, the nature of the aquifer, which is an open sand and gravel aquifer, doesn’t really fight back against that type of pressure. We really expect it to flow smoothly into the aquifer from this gravity fed injection.”
In the meantime, the team will be working on a method for treating the mass of the plume. Katzman discussed various options under consideration.
In one option – called pump and treat – extraction is targeted on the “bulls eye” of the plume in order to get the mass out of the ground.
“But there are far more failed cases of pump and treat programs in the country than successful ones, especially for large, complex plumes,” Katzman said. “It doesn’t mean that pumping in the center won’t be part of a large package of remedies that we bring to the table. We’re not real confident it would be the only solution.”
LANL does plan to drill an extraction well to the center of the plume this year in order to assess how it responds. According to Katzman, a common failure in pump and treat is that it initially removes high concentrations of chromium, but as pumping continues the chromium concentration decreases and the operation loses efficiency.
Other options under consideration include in situ treatments.
“Chromium responds very well to different types of chemical and sometimes biological treatment to convert it from hexavalent chromium to trivalent chromium, and it’s quite simple to do in a test tube in a laboratory,” Katzman said. “The difficulty is going to be figuring out how to deploy it in the field at 1,000 feet deep.”
In situ treatment has been done in the field both successfully and unsuccessfully. The lab is evaluating those results to try to determine why successes or failures occurred.
Katzman explained some of the difficulties of in situ treatment.
“People jokingly call in situ groundwater remediation contact sport, because basically, if you can get the amendment that you would be putting into the ground to do the work to go the places you want it to go to make contact with the contaminant, you’re in great shape,” Katzman said.
For example, a well used to add a chromium-neutralizing amendment may only have a 50-foot radius.
“So you can imagine how many wells we would have to have in order to create curtains where nothing gets by,” Katzman said.
Another option being explored is feeding naturally occurring microbes in the aquifer so they multiply, consume more oxygen and produce conditions that convert hexavalent chromium to trivalent chromium.
Katzman provided a timeline for the interim remediation, which began in 2015 with obtaining approvals and permits from the New Mexico Environment Department and completing a NEPA plan.
Full time hydraulic pumping should commence in May. Four to five injection wells will be drilled between May and December, and piping infrastructure for conveying the treated water to the injection wells will be designed and installed between June and October.
A new extraction well focused on plume center characterization will be constructed in the center of the plume and begin operations in May.
The goal is to have all permits in place and begin interim remediation sometime in November.
Katzman promised to update the board on any new developments.